Master control for electric seismographs



April 20, 1943. E. J. SHIMEK 2,317,334

MASTER CONTROL FOR ELECTRIC SEISMOGRAPHS I Filed Feb. 21, 1940 v 2Sheets-Sheet i [iv/A5751? con reoL if; RECORDEA? I [K I LAh'fi A 1A 2Tmrhru 2 Z T/ME April 20, 1943. E. J. SHIMEK MASTER CONTROL FOR ELECTRICSEISMOGRAPHS 2 Sheets-Sheet 2 Filed Feb. 21, 1940 Patented Apr. 20, 1943UNITE s'mres MASTER CONTROL FOR ELECTRIC SEISMOGRAPHS of New YorkApplication February 21, 1940, Serial No. 320,132

4 Claims. (Cl. 177-352)' This invention relates to electric seismographsand more particularly to a method and apparatus for controlling the gainin amplification as derived from a thermionic tube amplifier throughoutthe period of time during which seismic waves which have been created inthe earth's surface are being detected and amplified.

Another feature of this invention resides in the provision of meanswhereby a geophone that is used to detect waves which travel verticallyto the surface from the explosive charge, can be automatically cut outof the circuit so that the trace on which the vertically traveling waveshas been recorded, can be used to record signals which have beengenerated by one of the geophones forming the spread.

Heretofore, it has been necessary to use a separate trace on theseismogram for recording the uphole or vertically traveling waves. Thisdifiiculty has been overcome by the provision of a circuit that isenergized by the time break for operating a delayed acting relay todisconnect this geophone, which has been connected in parallel with oneof the geophones forming the spread.

A number of methods for controlling the gain in amplification as derivedfrom athermionic tube amplifier while amplifying seismic waves are knownin the art. These methods, however, have not proven entirelysatisfactory, since they did not control the amplificationcharacteristics of the amplifier in such a manner that all the necessarydata from a single spread, when shot in-one direction. could be recordedon a single seismogram. 'The master control forming the subject matterof this application makes such a record possible. The gain inamplification as derived from the amplifier is allowed to remain at amaximum value until signals corresponding to the first direct travelingwave energy have been recorded. Then the gain is automatically reducedto a predetermined value so that shallow reflections carrying highenergy can be recorded on a limited width of recorder strip. The gain inamplification then is caused to increase with time -as deeper and deeperreflections are detected andamplified.

Therefore, the primary object of this invention resides in the provisionof a master control for electric seismograph amplifiers. in conjunctionwith auxiliary circuits, by means of which it is possible to record allthe data required from a single spread when shot in one direction on asingle seismogram.

Another object of this invention resides in the provision of meanswhereby an uphole geophone can be cut out of the circuit after thevertically traveling waves have been detected at the surface.

Still another object of the invention is the provision of means forcontrolling the gain in amplification as derived from an electricseismograph amplifier in such a manner that it will remain at a maximumuntil signals corresponding to the first direct traveling seismic waveshave been recorded, after which it is immediately reduced to apredetermined low value and then automatically expanded with time insubstantially inverse proportion to the envelope of the detectedsignals.

Another object of this invention is the provision of means, initiated bythe detonation of the explosive charge, for placing the master controlin operation.

This invention also contemplates a master control for controlling thegain in amplification as derived from a plurality of amplifiers.

Other objects and advantages of the invention will be apparent from thefollowing detailed description when considered in the light of thedrawings in which:

Figure 1 is a diagrammatic illustration of a complete reflectionseis'mograph system showing the location of the geophones relative tothe explosive charge;

Figure 2 is a composite circuit diagram showing the invention formingthe subject matter of this application incorporated in the circuits ofan electric seismograph;

Figure 3 is a curve which has been plotted with negative suppressor gridpotential as ordinates and time as abscissae, showing the manner inwhich the suppressor grid potential as applied to a thermionic tube iscaused to vary to effect the desired variations in the gain inamplification as derived from the amplifier.

Figure 4 is a second curve that has been plotted with time as abscissaeand negative suppressor grid voltage as ordinates showing in addition tothe disclosure of Figure 3. an additional step in the voltage control;

Figure 5 is a curve which has been plotted with ain in amplification asordinates and time as abscissae showing the manner in which the gain inamplification is caused to vary while recording seismic waves.

Referring to the drawings in detail, particularly Figure 1, there isillustrated the usual seismic survey operation using the reflectionmethod by means of which data can be recorded from which to compute andplot profiles of the subsurface geological strata. These data are in theform of wave travel velocities, the waves bein created in the earthssurface by the detonationmeasuring the velocity of wave travel in theunconsolidated weathered surface layer or the earth. Various detailedmethods of accomplishing this result are well-known to those skilled inthe art and need not be discussed here.

The sequence of events occurring at the geophone after the detonation ofan explosive charge is first the detection of the first direct travelingwaves, followed by the detection of waves which have been reflected fromthe interfaces of substrata that it is desired to profile.

The geophone G can be of any type adapted to convert seismic wavesimpinging on it into electrical signals. These electrical signals thatare generated by the geophone are amplified through as many stages ofamplification as required and recorded by means of a recordinggalvanometer on a sensitized paper or photographic film in the form-oftraces, there being a trace for each geophone station in the spread.

Seismic surveying by the reflection method depends upon an analysis ofthe records of the waves recorded and the determination of the instantof arrival of these waves at the geophone or geophones. Additionally,in'order to make corrections for weathering to the times of arrival ofthe reflected waves, it is necessary to determine the instant of arrivalof the first direct traveling waves to reach the geophone or geophones.This means that the record must have an indication of the instant ofdetonation, some means for indicating on the record the passage of time,a clear record of the instant of arrival of the first direct travelingwaves and the instant of arrival of the succeeding reflected waves. Thedirect traveling waves and those waves which have been reflected fromrelatively shallow interfaces carry a high amount of energy and if thesensitivity of the apparatus was allowed to renot go of! of the recorderstrip, then automatically increasing the sensitivity of the apparatus asthe energy of the reflected .waves from the deeper interfaces decreases,so that all of the sig nals representing reflected wave energy ,will berecorded at substantially the same amplitude, regardless of the depth ofthe interface from which they have been reflected.

As an aid to the determination of the velocity of wave travel in theweathered layer, a geophone is placed adjacent the mouth of the shothole for detecting waves traveling vertically upward from the explosivecharge. The record from a geophone so placed heretofore has beenrecorded on a separate trace. In a recording galvanometer having apredetermined number of vibrating elements, this procedure wouldobviously limit by one the number of geophone stations that could beused in the spread. This difficulty can be overcome by connecting theuphole geophone in parallel with one of the geophones in the spreadthrough the medium of means whereby it can be disconnected from therecorder after it has served its purpose. The other geophone then will.

function in the normal manner. The result would be that one of thetraces would have not only the usual recorded data that would be similarto that of the other traces but would have in addition thereto a recordof the arrival of the uphole waves at the geophone that was located nearthe mouth of the shot hole.

Referring to Figure 2, the explosive charge S is detonated in theconventional manner by a blaster B. Rupturing the blaster circuit by thedetonation of the explosive charge generates a transient voltage in thecircuit that will pass through the transformer T1 to the grid circuit ofa gas triode tube ID. The grid of the gas triode tube I0 is negativelybiased by the battery II to prevent it from firing in the mannerwell-known in the art. The resistance I2 is placed in series with thebattery I I to prevent short-circuiting the battery when the switch I3is closed for test purposes. The transient voltage from the transformerT1 causes the negative potential on the grid of the gas triode to becomeless negative to a point where the grid no longer has control and platecurrent will immediately begin to flow in the plate circuit; the switchl4, provided for test purposes, is normally closed. The plate currentfrom the gas triode flowing through the resistance produces an IR dropacross it. This voltage, by means of the conductors I6 and I1, I8 andI9, is conducted to the input transformer T2 of the amplifier A throughwhich it passes to reach the recording galvanometer R, where it isrecorded as an indication of the instant of detonation of the explosivecharge S. The plate current from the gas triode also passes through theresistance producing an IR. drop across this resistance that serves asthe charging potential for the condenser 2|. The voltage from thiscondenser, by means of conductors 22 and 23, operates a multiple'contactrelay 24. The capacity of condenser 2| and the value of resistance 20 isso selected that ample time will elapse between the instant ofdetonation of the explosive charge, which coincides with the time thatthe condenser begins to receive its charge, and the time at which therelay 24 operates to disconnect the geophone 25from theinput transformerof the amplifier.

' The operation of the relay 24 accomplishes this tional.

by opening contacts f-g. It will be noticed that Additionally, thevarying potential when applied to an even number of stages in theamplifier, will block out distortion produced by varying the potentialon these grid elements.

The varying suppressor bias potential is supplied by a novel circuitarrangement, to be described hereafter, whose operation is initiated bythe relay 24. Before the detonation of the explosive charge S, the relaycontacts ae, h--i and fg are closed, and contacts be are open. With thiscondition existing, the tube 21 is blocked by the potential that issupplied by the battery 28. With contacts be open, the grid 29 of tube30 is at cathode potential but grid 3| of tube 30 with contacts a-eclosed, is biased by means of the batteries 32 and 33 to a pointconsiderably beyond the cut-off value by proper choice of thesevoltages. With this condition existing, there will be no plate currentflowing in the plate circuit 34 of tube 30 and therefore no dropproduced in the resistance 35. As a result, tubes 36 and 3'! in theamplifier will operate to produce maximum gain. This condition existsuntil after detonation of the explosive charge and the condenser 2|becomes charged to the relay actuating potential. When this value hasbeen reached, relay 24 will operate to open contacts a-e, bi and f--gand close contacts be. When contacts a-e open, the charging potential isremoved from condenser 38 and it begins to discharge through theresistances 39 and 40 to cause the negative bias on grid 3| of tube 30to vary toward zero as the condenser 38 discharges. The rate at whichthis bias be comes less negative and eventually reaches zero -iscontrolled by the resistors 39 and 40 and is rapid relative to the rateat which the potential of the condenser 4| changes. However, until theinstant that the potential on grid 3| reaches the point of cut-01f froma point below cut-oif,'the tube 30 is still blocked and there will be novoltage applied to the suppressor grids of tubes 36 and 31 in theamplifier. The time required for this potential to reach the cut-offpotential is so selected by varying the resistance 40 that signalscorresponding to the first direct traveling Waves that have beendetected can be recorded before any suppression of the gain inamplification as derived from the amplifier is effected.

Since at the time contacts a-e opened, the contacts be closed, thepotential as supplied by the condenser 8| to the grid 29 of tube 36begins to become more and more negative at a rate determined by theresistance 42. This change in potential on grid 29 though is slow ascompared to the rate of change of potential on grid 3|, but this changein the grid voltage on grid 29 will eventually block the tube and bringthe potential on the suppressor grids of tubes 36 and 31, which waspermitted to increase as the potential on grid 3| became less negative,back to zero at which time the amplifier is again operating at fullsensitivity.

With the circuit described thus far, the potentials on the suppressorgrids of tubes 36 and 31 will be caused to vary in the mannerillustrated in Figure 3. This curve has been plotted with negativesuppressor grid potential as ordinates and time as abscissae. From thetime to to ii, the bias on grid 3| of tube 30 is becoming less nega tiveto the point of cut-off. From the time ii to t. the potential on grid 3|is becoming less negative and the potential on grid 29 is becoming morenegative but at a much slower rate of change than the potential on grid3|. This will result in a suppression in the gain in amplificationduring the time 1 to 2. From is to t: the biasing potential on grid 29is controlling and this potential will become more and more negativeuntil tube 30 has again become blocked. During this time t2 to t: thenegative potential supplied to the suppressor gridsis becoming less andJill less negative and will reach zero when tube 30 has been blocked bythe potential on the grid 29.

Due to the fact that the energy carried by the waves that have beenreflected from deep interfaces is relatively low, it is desirable tohave the gain in amplification as derived from the amplifier from acertain time on to the end of the record. increase more rapidly. Thisresult is accomplished in the manner hereafter described.

During the time when the above described sequence of events wereoccurring, the voltage across the condenser 43, due to the fact that thecontacts h i were opened by the operation of the relay 24. has beenincreasing positively. but at this instant the tube 27 is still blocked.This positive increase in the voltage across condenser 43 is produced byits discharging through the resistance 46. Therefore, the voltage, dueto the absence of plate current in the plate circuit 45 of the tube 21,across the condenser 38, remained at. zero potential. mined by thechoice of resistance 44, the tube 21 becomes conductive and the voltageacross the condenser 38 begins to increase negatively. This rate ofchange will be reflected by means of the grid 3| in the tube 30 to causethe rate of change of suppressor grid bias that is being applied totubes 36 and 31 in the amplifier to suddenly increase in such a mannerthat it will approach zero much more rapidly. The curve in Figure 4represents the manner in which the suppressor grid voltage, as suppliedto the tubes 36 and 31 in the amplifier, is caused to behave over the'period of time during which seismic waves are being recorded. Thiscurve, like that in Figure 3. has been plotted with negative suppressorgrid potential as ordinates and time as abscissae, the periods of timeto to t1 and ii to it: being the same as described in connection withFigure 3. In Figure 3. however, the period of time from t2 to is islimited by the time at which the tube 21 becomes conductive. The time t:to t4 represents variation in suppressor grid potential from the timethat tube 21 became conductive to the time that tube 30 was blocked andthe suppressor grid potential again became zero,

A variation of the gain in amplification as derived from tubes 35 and.31 during the same periods of time indicated in Figure 4 would be asshown in Figure 5. During the period to to 11. while the suppressor gridpotential is zero. the amplifier is operating at maximum sensitivity. Asthe suppressor grid potential becomes more and more negative during thetime tl to n, the

gain in amplification is suppressed. However, as

the suppressor grid potential varies toward a less negative value duringthe period 2 to is, the gain in amplification is caused to increase andfrom the time t; to is. during which the negative However, at a timedetercontacts 48'; can be used to simultaneously control the gain inamplification as derived from a plurality of amplifier channels.

The control voltage has been described as applied to the suppressorgrids of the pentode type tubes. It is obvious to those skilled in theart that this varying potential may equally as well be applied to thecontrol grids ofothertypes of tubes.

In the above specification the term uphole waves has been used toidentify those waves which travel vertically upward from the explosivecharge to the surface of the earth. The g'eophone which detects thesewaves is referred to as the uphole geophone.

The detailed circuits of the various elements used in the process andapparatus above have not been specifically described inasmuch as theyare commonly known and may be varied within relatively wide rangeswithout departing from the principles of this invention.

I claim:

1. A method of recording seismic waves that comprises the steps ofcreating seismic waves in the earth's surface at a point verticallyspaced below the surface, generating a signal coincident with thecreation of the seismic waves, recording the generated signal on thetrace of a seismogram as an indication of the instantof creation of theseismic waves, detecting the first impulses of waves which have traveledvertically upward from their point of creation, amplifying and recordingthe first impulses on the same trace on which the signal indicative ofthe creation of the seismic waves has been recorded, renderinginoperative said detecting means after the first impulses of the wavestraveling vertically upward have been detected, amplified and recorded,detecting waves which have traversed paths substantially directly fromtheir point of creation to a plurality of detecting stations that arecollaterally spaced from said point, amplifying and recording the firstimpulses of these waves on separate traces one of which being the tracerecited above, reducing the gain in amplification imparted to thedetected direct traveling waves after their first impulses have beenrecorded, detecting, amplifying and recording on the separate traces andthe first-mentioned trace waves which have traveled downwardly fromtheir point of creation and have been reflected from the interface ofthe substrata, increasing the amplification imparted to the detectedrefiected waves at a predetermined rate for a predetermined time,independent of signal strength, then accelerating the rate of increasein amplification imparted to the detected reflected waves independent ofsignal strength during the recording of the remainder of the seismogram,whereby the reflected waves will be recorded with substantially -equalamplitude.

2. An apparatus for recording seismic waves comprising in combination, aplurality of means for detecting said waves by generating electricalsignals corresponding to them, separate means for respectivelyamplifying and recording said signals in coordination with time, asingle gain control for controlling the gain in amplification impartedto the signals by each amplifier, said gain control comprising means forreducing the gain in amplification after signals corresponding spondingto direct traveling waves are recorded with maximum gain inamplification and signals corresponding to detected reflected waves arerecorded with substantially equal amplitude.

3. An apparatus for recording seismic waves comprising in combination,means for detecting said waves by generating electrical signalscorresponding to them, means for amplifying and recording said signalsin coordination with time, a gain control for controlling the gain inamplification imparted to the signals by said amplifier, means forinitiating the operation of the gain control a predetermined period oftime after the creation of said seismic waves, said gain controlcomprising means for reducing the gain in amplification after signalscorresponding to detected direct traveling waves have been amplified andrecorded, means for thereafter increasing the gain in amplification at apredetermined rate throughout a predetermined period of time, andseparate means for accelerating the rate of increase of the gain inamplification throughout the period of time while the remainder of therecord is being recorded, whereby signals corresponding to directtraveling waves are recorded with maximum gain in amplification andsignals corresponding to detected refiected waves are recorded withsubstantially equal amplitude.

4. An apparatusfor recording seismic waves comprising in combination,means forming a conventional spread for detecting said waves bygenerating electrical signals corresponding to them, means foramplifying and recording said signals in coordination with time, anadditional detecting means adapted to be positioned adjacent the shotpoint for detecting waves that have traveled vertically upward fromtheir point of creation, means'for connecting said additional detectingmeans to amplifying means common to one of the first mentioned detectingmeans, a gain control for controlling the gain in amplification impartedto the signals by said amplifier, common means for rendering theadditional detecting means inoperative and for initiating the operationof the gain control a predetermined time after the creation of theseismic waves, said gain control comprising means for reducing the gainin amplification after signals corresponding to detected directtraveling waves have been amplified and recorded, means for thereafterincreasing the gain in amplification at a predetermined rate through apredetermined period of time, and means for accelerating the rate ofincrease of the gain in ampl fication throughout the period of time Whie 'the remainder of the record is being recorded, whereby signalscorresponding to direct traveling waves are recorded with maximum gainin amplification and signals corresponding to detected refiected wavesare recorded with substantially equal amplitude.

EDWIN J. SHIMEK.

